


First Heat

by FullOfBoredom



Category: Undertale (Video Game)
Genre: Classicberry - Freeform, Come Inflation, Knotting, M/M, Mating Cycles/In Heat, Painful Sex
Language: English
Status: Completed
Published: 2021-01-24
Updated: 2021-01-24
Packaged: 2021-03-16 03:07:46
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,920
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28949418
Author URL: https://archiveofourown.org/users/FullOfBoredom/pseuds/FullOfBoredom
Summary: “Sans! You can’t hide this from me forever! I…!” Blue felt his eyes tearing up. He dropped his voice down to a whisper. “I just want all of you. Don’t you trust me with that?”He heard a thud on the other side, like Sans had collapsed against it.“I…don’t…trust myself.” Each word came out grated, like it was a struggle to say.“I trust you. Isn’t that enough?”
Relationships: Sans/Sans (Undertale)
Comments: 5
Kudos: 65





	First Heat

“Sans?” Blue knocked hesitantly on their bedroom door, where his boyfriend was currently hiding.

“Go away Blue…” A small groan and other tiny sounds filter through, muffled by the door and probably blanket Sans was hiding under.

Blue sighed and leaned against the frame, sliding down to sit by the door. They’d been having this fight for awhile now, and Blue was positively at wit’s end!

Sans and Blue started dating a few months ago. It was super unexpected, but the comedian had made him laugh, with a PUN of all things, and then continued to do so the entire night. As the time ticked away, Sans had told him about himself a little more, more than the general knowledge he had of the other’s timeline, cracking jokes the whole time. Blue laughed along but also talked fondly about his dreams and his home. They had exchanged numbers, promising to keep in touch.

After that, it’d been a lot of ‘not dates’ which ended with them awkwardly standing by Blue’s door, both unsure of what to do until the last one. Blue just went for it, clanking Sans right on the mouth. He spun around and spoke to his boots.

“I really like you Sans! Do you want to come inside and watch a movie? I don’t want this date to end!” Blue turned back to face him, caught off guard by the very cool blue blush on Sans’s face. More so by the hands that held his before a gentle clank on his forehead.

“I like you too. I’d love to.”

Fast forward a few weeks, and they were at each other’s place every night. They cuddled on the couch, Sans making commentary over movies and shows, Blue defending them but also ending up in giggle fits (because Sans devolved to tickling if he couldn’t grab Blue’s attention).

It only took a few tickle fights before things had gotten…physical. The first time had been pretty innocent. Sans dropped a “Stars you’re cute” before kissing him hard. Blue wrapped his arms around his neck, and Sans took the cue to lean in to get an actual taste.

Blue blushed thinking about it. That had been…an experience. But that wasn’t important right now!

The real trouble was that while he and Sans had fun here and there, they didn’t spend any of his heats together. Sans completely closed himself off when he went into them. Blue tried all manner of begging, but it got him nowhere. 

Even more frustratingly, Sans practically leapt at the chance to help Blue with his. Sexually all give, and no take, though Blue often just rode himself out, considering the energy difference. They had already experienced one of Blue’s heats since moving in together, but this was Sans’s first in the house. Blue had hoped the last one had been the last one his boyfriend spent alone.

“Sans! You can’t hide this from me forever! I…!” Blue felt his eyes tearing up. He dropped his voice down to a whisper. “I just want all of you. Don’t you trust me with that?”

He heard a thud on the other side, like Sans had collapsed against it.

“I…don’t…trust myself.” Each word came out grated, like it was a struggle to say. 

“I trust you. Isn’t that enough?”

The long drawn out silence almost made Blue leave, ready to give up on this, and possibly on them at this rate, but right as he went to stand, the door pulled back, and he was pinned to the floor.

The first thing he noticed was Sans’s eyes. They were blown wide, glowing blue and yellow, flashing and searching Blue’s face. His whole body felt hot, blue glow coming out from every piece of clothing. Blue’s hands were pressed into the carpet and Sans straddled his hips. Blue felt a shiver up his spine as Sans leaned down to his skull.

“You got one chance little Blue.” The smell was overwhelming. “If you choose this, it doesn’t end until I say so. I’m…not strong enough to stop.” Sans took a deep breath. “You smell delicious.”

Blue’s body started to surge up, wanting to fight being held down, but his soul pounded at the thought of submitting, and he just writhed.

“I’m all yours Sans.”

“All MINE.” Blue felt Sans’s tongue lick up his neck vertebrae. He held in the whimper, but he heard an uncharacteristic growl before the hard chomp on his collarbone. That, he didn’t hold in.

“AHH!” It ached, and Sans kept nipping around the probable mark. With the bite hot and angry on his shoulder, Blue whined and pulled at his ‘restraints.’ Sans didn’t move his head away, he forced Blue’s wrists under one hand instead of two. Blue’s head was forced up to fit his arms under it, and Sans used his free hand to separate Blue’s legs to switch from straddling to settled between them.

“How attached are you to these clothes?” He trailed his fingers over his clothed ribcage. Blue shivered and breathed through the light panic.

“Not…very?” A bone attack lodged itself between his radius and ulnas, freeing up Sans to grab the shirt with two hands.

He ripped it clean down the center up to the collar.

“Sans!” Blue struggled, but the attack held, it’s power higher than his from this angle. Were… his teeth sharper? Sans ducked his head and bit the collar in half, smirking triumphantly when he pulled back. His canines were more fang-like than usual. How intense were his heats that it physically altered him? “What are yo-S-S-STARS!”

Sharp phalanges raked across his ribs, curving inside to the more sensitive sides. There’d be marks for weeks. 

“I wanted you so bad, you look so good like this, mine, mine, mine.” 

He pulled the pants and boots off a little nicer, not trying to pop out his hips, but not patient. Blue squeaked at the Sans forcing three fingers in right away.

“You’re so tight.” Blue panted as the fingers worked quickly into him, spreading him further open. Sans usually took so much time, also his fingers weren’t usually pointed. It wasn’t sharp enough to puncture his ectoflesh, just a bite of pain where Sans’s fingers bottomed out inside him. 

“SA-AA-AAA-NSSSS!” Blue was naked, aching, pinned to the floor of his hallway, with his heat-crazed boyfriend roughly fingering him. Overwhelmed, and out of his depth, he opened his mouth to tap out. “PUT IT IN ALREADY!”

His eyelights went out, face getting very bright. Sans just moved up to look at his face from biting all over his chest.

“First of all…”Sans grinned wildly, flexing his finger as far wide as he could stretch. “You couldn’t take me right now without significant pain.” Blue swallowed hard. “Secondly, since when were you the one in control?” But the fingers did retreat. And something much bigger lined up.

“Oh Stars…” 

Sans’s cock transformed to something heftier, with a thick knot at the base. Blue realized the tip pressed against him would already be a stretch, but by the time he thought to struggle, Sans already had a firm grasp on his pelvis.

“Since you insisted, how ‘bout a taste?” 

“MWEEEEEEEEEEEEEEH!” His voice climbed to just short of a scream. God, being split in half would probably be less painful. His eyes teared up intensely, holding back at soft gasps of pain in place of full on crying. He chanced a glance downward. 

Sans was only half inside of him. The knot was even wider. There was no way.

Sans looked half crazy, grinning softly, eyes enamored with Blue’s struggle. He shifted his hips back and forth slowly, cock twitching while Blue had opened his mouth in a silent cry. And then fingers slid across his clit and pulled him even tighter.

“If you cum for me, I’ll stretch you out properly.” Blue had devolved to actual sobs, but there was something in being stretched to the absolute limit, a pleasurable edge to the pain, though the pain was immense. Sans was beyond listening, so Blue couldn’t ask for what he wanted or needed, but Sans kept rubbing at a steady pace, and Blue was working up to a peak that would work. He just needed something to really force it over to distract him from how split he felt.

Blue held his breath, and tilted his head to the side, leaving the other side of his collarbone wide open.

And Sans pounced on the opportunity.

“SANS!” Blue moaned hard at the bite, finally, finally, coming around the thick head inside of him. He almost cried when the huge cock was finally out of him.

“Let’s move to somewhere a little more private.” Blue could barely feel being lifted into the room, but he did feel better being laid on a bed. He could hear the door close (trapped in the lion’s den) and Sans was by him again, all his clothes gone. Just as fast, his face vanished to feel something hot near his pussy again.

“Too soon!” But Sans plunged his tongue in anyway. He licked and sucked at the sore lips, nose rubbing up into his clit again.

Blue tried to call out, say something, but he was overstimulated to the point where time was passing in a blur. All of his nerves were on fire, his soul constantly lighting up the room, too close to the edge too soon and unable to peak again. Sans just kept working at him, and Blue wanted to cry again, feeling very lost. Where was his Sans? Not that this was the most unpleasant, but nothing felt familiar about this Sans. His lazy fun boyfriend was nowhere in this crazed copy.

Eventually, between the softer ministrations and heat scent, his body relaxed enough, and he found something enjoyable in the stretching. Soft moans begans to fill the air, and Sans took it as some type of signal because he removed his face from his pussy, and replaced it immediately with his fingers. He started with four. It wasn’t as uncomfortable as he thought.

Sans nibbled up and down his femurs. He was murmuring something, possibly for a long time, but only now was he quiet enough to hear it.

“Just want to devour you, so delicious, gunna take all of me…”

He had missed the fifth finger, but he could feel the entire hand working its way inside. 

“Your whole hand - mweh-hah - fits?” Sans hadn’t looked up but sounded so awed.

“Babe, I could ulna-st fit my whole arm.” And maybe it was the whole ridiculousness of it all, but Blue chuckled a little, because why not at this point? Sans looked up at him this time, and…

Oh.

His eyelights had gone into hearts and his hand sunk as far as it could go. Blue arched his back, feeling the bones of Sans wrist and the top of his arm sink into him.

“Oh stars Sans! I-I didn’t think I could-”

“Of course you could babe. You’re MINE, you can take all of me.” He slowly twisted and pulled on his arm, finally content and pulling it fully out. So that’s what being empty felt like.

Sans licked his fingers clean, humming while knowingly staring at Blue’s face. His tongue lolled out of his head, savoring the flavor. He went in for the first kiss since this whole thing started. His tongue tasted of himself and Sans’s heat. Half wrapped around his own, Blue sighed into it.

Blue clutched at his Sans desperately, needing an anchor, feeling cored like an apple, but Sans had only been sensible as a means to an end. He needed Blue relaxed, and when he opened his eyes, his hands were bound together pressed up against Sans chest.

“It’s been long enough.” Blue felt like he’d been betrayed, but he’d just forgotten how this had started. He did fight Sans trying to turn him over onto his hands and knees. He wiggled and fought, adjusting his hips to try and get Sans slid into him.

“Don’t make this harder than it needs to be.” Blue came up with a distraction on the fly, rubbing up against the thick member.

“I think this is as hard as it gets.” Sans froze from the pun. That second was all he needed.

“OoooooooooH.” The tip caught, and Blue just slid down. He stopped right at the top of the knot.

Sans eyelights vanished, turning to look at his crafty boyfriend, who had his bound hands tightly grasping his ribs. 

“You know this’ll be harder on you.” His hands rested on Blue’s pelvis, stroking over the arches, but the weight of them pressing the knot up against Blue’s entrance. Blue met his gaze head on.

“I won’t let you run from me Sans.” And he kept that stare up, rocking his full weight into Sans lap, the knot slowly sinking home. 

“Shit.” Sans groaned. “You’re so tight.” The dark gravelly tone crackled. Their hips rocked together. “I need this, I need you.”

“It’s so much!” Blue rocked with him. If he’d never been more empty before, he’d never been so full now.

“You have to be mine. I-” Sans lifted him an inch and guided him back down. He wrapped his arms around Blue, pulling them ribs to ribs, gently kissing his forehead. “I can’t stop.”

Blue squeezed around the knot inside of him, panting in time with Sans’s moan. 

“I-” Blue looked up, on the verge of tears. “I don’t know if-” Another small thrust. “-if I can handle-” Another. “-much more.” Three in short succession.

“You can take it.” Sans switched to longer thrusts. Blue’s hands tightened around Sans’s ribs, leaning back into them. Soothing circles rubbed into Blue’s pelvic crests, Sans escalated into faster and harder thrusts. Soon, each thrust pulled almost all the way out, only to slam back into Blue, knot fully hilted. Blue’s vision was blurring at the edges, his body gradually aching while he stretched further and further with the knot gaining size. “I can’t wait to knot your sweet little pussy and make you mine forever.”

“Please! Knot! Me!” Blue could feel how close Sans was getting, and hoped his endurance would hold out just a moment longer. He was getting swept away with his insides being shifted around. Sans hadn’t really talked to him about the far future, just about the immediate. He whimpered as his orgasm barreled down towards him, trying to focus on whatever Sans said next to distract himself. 

“I love you so fucking much, take my fucking knot baby.” 

Blue felt one last push, and suddenly Sans was completely still inside him. The knot just kept expanding, until Blue shuddered and shook and tightened painfully around it, then he was gone. 

“Sans!” Every bone in his body rattled. All thoughts he had had over the last 2 hours blurred away. All he could feel was his full body, warmth flooding his insides, and his soul settling.

Sans was holding him when he came to, hands already freed. He went to shift, but his body pulled back. The knot was stuck inside him.

“Sorry…” Sans just held him close, body sated, but his face said his mind was far from it. 

Blue looked down. His entire body was full of blue magic, held in by a knot Blue was still surprised could fit without tearing him in two. Well, Sans couldn’t run from this conversation now. 

“What was that?” Best to go simple. He rubbed his hand over his swollen stomach, all the excess magic trapped inside.

“I…have really possessive dominant heats.” Sans avoiding looking at him. “You deserve better treatment than…that.” He reached for a blanket to pull around Blue when he shivered, but only managed to get his hoodie.

“And what did you mean by make me yours forever?” Sans really avoided his gaze there. “I didn’t think that required covering my entire insides in your magic.”

“Jesus Blue. I just meant it doesn’t feel like you’re really mine until we spend a heat together. Until I…knot you.” Sans blushed, embarrassed but hesitantly rubbed a hand over Blue’s stomach. Then so softly, it might’ve been unintentional. “You’ll look so cute pregnant.” Oh definitely not meant to be said out loud, they both couldn’t look at the other.

The heavy silence carried until the knot shrunk enough for Blue to be freed. He made a mess of the blankets, but laundry wasn’t even a thing right now. He laid back, aching but warm in Sans’s hoodie.

“You wanted kids?” Sans had been cleaning up while Blue rested. He tossed the blankets in the hamper.

“I mean, I do, but it’s not a dealbreaker if you didn’t.”

“I do too. But that’s for another day.” Sans sat on the bed’s edge, looking at Blue hiding under the hood. “I love you Sans. You should’ve just told me. This wasn’t bad to ride out, but a little warning would’ve been nice.” Blue curled into the hoodie, hugging himself with it. Sans laid down and wrapped his arms around both of them.

“I love you too. Forgive me?”

“Tibia honest, I already had.” Blue could feel Sans hug him tighter, the light chuckle, and a sigh of relief, before he slid his arms into the sleeves and cuddled into him. It was time to rest, and with Sans covering them in a blanket and a quick peck, they drifted off.

**Author's Note:**

> Written as part of Kinktober.


End file.
